Tankless electric water heater electrical?

[mooker82]

We are redoing our laundry and are thinking of replacing our water heater with a tankless electric due to space concerns(gas is not an option). We have electric everything (range, water heater, ac, dryer, heat pump) and I'm concerned about the power requirements of the tankless. I've had an electrician give me a quote and he said I will be ok given that I have 200 amp service. He did not do any load calculations though. The tankless requires 2 60 amp breakers and takes 24kw. It seems to me I will be pushing the limits of my 200 amp service based on the calculations I've done from on online sources. The house is 1400 sqft. Can anyone help? I'm definitely not an electrical expert and hate to call guys out for quotes if its not feasible.

 

[LandCruiserPhil]

Normally tankless work off of demand so under the hardest conditions it will pull 100A. Aside from the electrical install cost have you run the numbers as for cost? I only ask because I have had a hard time justify electric on demand water heaters.

You will be close but chances are you we be fine as long as your service is not an older FPE or Zinsco. I would make sure all you 220v breaker are full size and you have 4 full spaces available for the new breakers. High amperage thin breakers create a lot of heat and tend to cause buss problems over time.

 

[mooker82]

Normally tankless work off of demand so under the hardest conditions it will pull 100A. Aside from the electrical install cost have you run the numbers as for cost? I only ask because I have had a hard time justify electric on demand water heaters.

You will be close but chances are you we be fine as long as your service is not an older FPE or Zinsco. I would make sure all you 220v breaker are full size and you have 4 full spaces available for the new breakers. High amperage thin breakers create a lot of heat and tend to cause buss problems over time.

We have the space in the box and he quoted running the 2 required 6 gauge wires. Given our situation the tankless even with the electrical is cheaper. If we go back tanked we will have to move plumbing, extend wires, and build an enclosure to relocate outside in addition to purchasing a new tanked model. It is likely the only time an electric tankless makes sense.

 

[LandCruiserPhil]

We have the space in the box and he quoted running the 2 required 6 gauge wires. Given our situation the tankless even with the electrical is cheaper. If we go back tanked we will have to move plumbing, extend wires, and build an enclosure to relocate outside in addition to purchasing a new tanked model. It is likely the only time an electric tankless makes sense.

Sounds like you have done your homework

Now the whole family can shower back to back

 

[decavo]

Keep in mind you will never pull 200 amps... the hot water heater, every eye on the stove plus the oven on, with the heat pump going, while washing and drying clothes, with the dishwasher going, every light in the house turned on, and every small appliance on all at the same time could cause a problem.... How often would that scenario happen????? I am an electrician and that is not how a service for a home is calculated... That is why you have a 200 amp service. One never uses everything all at once in a home... If you have space for the breakers then it will be an easy install...

 

[mooker82]

Keep in mind you will never pull 200 amps... the hot water heater, every eye on the stove plus the oven on, with the heat pump going, while washing and drying clothes, with the dishwasher going, every light in the house turned on, and every small appliance on all at the same time could cause a problem.... How often would that scenario happen????? I am an electrician and that is not how a service for a home is calculated... That is why you have a 200 amp service. One never uses everything all at once in a home... If you have space for the breakers then it will be an easy install...

This is the calculation I used. I hope it makes sense. I guessed on some appliances using figures found online since I can not find specs on mine without removing them.

1400 sq ft x 3w= 4200w
3 small appliances x 1500w= 4500w
1 laundry= 1500w
Range= 8000w
Dryer= 5000w
Dishwasher= 2000w
Refrigerator= 1500w
Total= 26700w

26700w - 10000w= 16700w

16700w x 0.4= 6680w

6680w + 10000w= 16680w variable use

16800w variable
+7000w AC and air handler
+ 24000w water heater
Total= 47800w

47800w/240=199.17amps

 

[LandCruiserPhil]

This is the calculation I used. I hope it makes sense. I guessed on some appliances using figures found online since I can not find specs on mine without removing them.

1400 sq ft x 3w= 4200w
3 small appliances x 1500w= 4500w
1 laundry= 1500w
Range= 8000w
Dryer= 5000w
Dishwasher= 2000w
Refrigerator= 1500w
Total= 26700w

26700w - 10000w= 16700w

16700w x 0.4= 6680w

6680w + 10000w= 16680w variable use

16800w variable
+7000w AC and air handler
+ 24000w water heater
Total= 47800w

47800w/240=199.17amps

Not correct ^^^ IIRC the water heater can go in the first set of calcs. Then only the 1st 10KVA is 100% and then 40% of the remaining then add in your other load and AC
Also there is an optional method per NEC that sometimes can also work in your favor.

 

[mooker82]

Not correct ^^^ IIRC the water heater can go in the first set of calcs. Then only the 1st 10KVA is 100% and then 40% of the remaining then add in your other load and AC
Also there is an optional method per NEC that sometimes can also work in your favor.

Perfect! I could not find anything related to calculating for a tankless. Based on this even using the full 24kw I am fine (falling under 140amps).

 

[LandCruiserPhil]

Perfect! I could not find anything related to calculating for a tankless. Based on this even using the full 24kw I am fine (falling under 140amps).

Being fine on paper is one thing but like decavo said unless you turn everything on full blast you should be OK. Remember the on-demand only uses the power needed for the flow, low flow = low electrical usage.

 

[mooker82]

Being fine on paper is one thing but like decavo said unless you turn everything on full blast you should be OK. Remember the on-demand only uses the power needed for the flow, low flow = low electrical usage.

Thanks! My only concern would be an AC running, hot shower going, while washing and drying clothes scenario which happens a good bit. Luckily our water temperature is high coming into the house and will not require a huge rise in temp.

 

[itbrokeagain]

Don't do it. You will not be happy with it. Gas thankless heaters do the job well. Electric is not worth installing IMO. They draw too much. Most are 3 phase 180 amp.

 

[fieldsken1]

As far as having enough room in the panel just run a sub panel

 

[mooker82]

Don't do it. You will not be happy with it. Gas thankless heaters do the job well. Electric is not worth installing IMO. They draw too much. Most are 3 phase 180 amp.

Have you used one of the newer ones? I know the power requirement is huge and for a large house they would not make sense. I am looking at the Stiebel Eltron Tempra 24 plus which is single phase 240 with 2 60 amp breakers. Everything I have read from users is positive. I'm not going into this thinking I will save money, just laundry room space. I'm actually expecting a slight rise in power and water bills since there won't be any cold neater running my wife out of the shower.

 

[re_guderian]

Thanks! My only concern would be an AC running, hot shower going, while washing and drying clothes scenario which happens a good bit. Luckily our water temperature is high coming into the house and will not require a huge rise in temp.

Check your rate plan as well. Capacity is one thing, but my kWh cost is partly based on my max peak usage per month. Not sure how common this is, but if I've got everything running at once, I pay more, hence load controller installed...

 

[redsonja]

Didn't see this mentioned yet. I'm a plumber so im not gonna address the power but the water. Unless u live in a state that doesn't get a winter the instant hot water heater will not get your water hot enough. The ground water temperature coming in can be too cold in a lot of parts of n America. An instant hot would work better as part of a re heat system where the water going in it is already warmed. But if u live in a sunny hot place it may be okay.

 

[mooker82]

Didn't see this mentioned yet. I'm a plumber so im not gonna address the power but the water. Unless u live in a state that doesn't get a winter the instant hot water heater will not get your water hot enough. The ground water temperature coming in can be too cold in a lot of parts of n America. An instant hot would work better as part of a re heat system where the water going in it is already warmed. But if u live in a sunny hot place it may be okay.

Per the charts the incoming temp is around 62. We have pretty mild winters in Charleston, SC. At 108 degrees for showers the tankless would flow 4.25 gpm and at 120 for everything else flow 3 gpm. This should be sufficient for us as the likely hood of 2 showers at the same time is slim, most laundry is cold washed, and the dishwasher has its own heating element.

My big concern is reliability. I know you have to flush yearly so I plan on using the special clean out style cut offs on both the hot and cold side. I have also looked at the Rheem units. My neighbors have one and have been happy with it. They have only had it 3 months though.

 

[itbrokeagain]

Didn't see this mentioned yet. I'm a plumber so im not gonna address the power but the water. Unless u live in a state that doesn't get a winter the instant hot water heater will not get your water hot enough. The ground water temperature coming in can be too cold in a lot of parts of n America. An instant hot would work better as part of a re heat system where the water going in it is already warmed. But if u live in a sunny hot place it may be okay.

I am also a plumber. I have a natural gas tankless in my house and I have installed at least 15 in the past couple years. In Cincinnati right now the incoming water is very cold but the tankless keeps up no problem. We can run 2 showers at once or we can run the dishwasher washing machine and a shower with no issues. Anyone that come to me for a tankless electric I talk them out of it. My yearly service includes a heat exchanger flush and I remove the burner and clean the flame rods. Cleaning the flame rods is very necessary.

 

[Azca]

The number 6s may be a problem. The 2011 NEC will not allow SE cable to be used at the 75c rating if it is installed in thermal insulation (which in most cases it will be). #6 at 60c is good for 55 amps. The NEC also requires that the branch circuit for an appliance be a minimum of 125% of the rating of the appliance.

There is another way to go about the load calculation. Call your utility and get a "high demand" from them. This is the highest amp draw you will have experienced in the last year. Take that value and multiply by 125%, then add the 24kw (also at 125%) to the load.

All of that said, you are VERY close to having a problem with nuisance tripping. That of course will only happen at the most inopportune time! Here in the Phoenix metro area we are also charged an increased cost of use during the day, depending on plan that can be from 8am all the way to 9pm. The extra fee is about 3 times the normal rate. Not sure if you have the same problem but if you do that should be factored into the equation.

My limited experience with this equipment is to stay away from the electric units. Gas is a much better option for this equipment...

 

[mooker82]

The number 6s may be a problem. The 2011 NEC will not allow SE cable to be used at the 75c rating if it is installed in thermal insulation (which in most cases it will be). #6 at 60c is good for 55 amps. The NEC also requires that the branch circuit for an appliance be a minimum of 125% of the rating of the appliance.

There is another way to go about the load calculation. Call your utility and get a "high demand" from them. This is the highest amp draw you will have experienced in the last year. Take that value and multiply by 125%, then add the 24kw (also at 125%) to the load.

All of that said, you are VERY close to having a problem with nuisance tripping. That of course will only happen at the most inopportune time! Here in the Phoenix metro area we are also charged an increased cost of use during the day, depending on plan that can be from 8am all the way to 9pm. The extra fee is about 3 times the normal rate. Not sure if you have the same problem but if you do that should be factored into the equation.

My limited experience with this equipment is to stay away from the electric units. Gas is a much better option for this equipment...

Thanks! I called my utility company and someone will call tomorrow that can hopefully provide me the information. They could only provide the max kw which was 1200kw. Our utility company currently charges a flat rate for residential, although I am sure they will change when they learn they can make more money by calculating another way. Gas is not an option. I do not have a line to my house. I have checked into it before but it is cost prohibitive.

Right now we have a tanked which is in the laundry. We are hoping to use the tankless so we can shrink our laundry room to expand a bathroom. If we go back to tanked we will have to move it outside, move plumbing, wiring, and build an enclosure.

I really appreciate everyone's input. Mud has an answer of everything.

 

[itbrokeagain]

If you do decide electric wont work what about propane?